Optical subtraction of images is an important processing operation used extensively in various disciplines interested in the extraction of difference information between two optical images. These disciplines include area surveillance, robotics, inspection and quality control, photography, microelectronics and photogrammetry. In most of these applications two records or images are available simultaneously, in the form of photographs, slides, prints, or other fixed media. However, comparison between a real time image and one that has been previously stored (on film, etc.) is of great significance.
Several techniques have been suggested in the past for optically obtaining a subtraction of, or registering a difference between, two images. A comprehensive review of previous image subtraction techniques appears in "Optical Image Subtraction", by John F. Ebersole, Optical Engineering, Vol. 14, No. 5, Sept.-Oct. 1975, pp 436-447. This paper provides an extensive review of image subtraction techniques using both coherent and incoherent based optical systems, with real time or delayed-time processing. Most techniques require the use of several sequential operations and possibly generation of intermediate image records. Generally these techniques suffer from insufficient dynamic range, poor signal-to-noise ratio, and lack real time operation capability. Some of the techniques require polarization of the sources illuminating the images to be compared. In addition, many of these techniques utilize quite complex optical apparatus which is not desirable in many commercial applications.
More recently an apparatus utilizing two liquid crystal light valves was demonstrated and disclosed in U.S. Pat. No. 4,124,278, issued to E. Marom and J. Grinberg Nov. 7, 1978. In this patent Marom and Grinberg disclose a subtraction apparatus in which two images, a positive replica of one input and a negative replica of a second input, are superimposed in registration by using two liquid crystal light valves. This positive-negative superposition results in subtracted features between the two images appearing, in real time, as either bright or dark regions on an average grey background. While image subtraction is performed in real time, the contrast is lower than desired for some applications.
Prior to that, in "Real Time Grid Coding and Interlacing for Image Subtraction" in Appl. of Holography and Optical Data Processing, Eds. E. Marom, A. A. Friesem and E. Weiner-Avnear, Pergamon Press, 85-91 (1977), Konforti and Marom also described a method by which two images to be subtracted are multiplied by complimentary coding functions (i.e. Ronchi rulings) and then recorded on a common medium. Subsequent filtering of the composite image reveals the difference on a dark background. In this system the subtracted image has very good contrast but the subsequent filtering necessary to reveal the subtraction is time delayed. Even adding a video camera and monitor to such a system to achieve real time display of the image subtraction is limited by the resolution of conventional cathode ray tubes which greatly reduces the usefulness of this technique.
What is needed then is a method and apparatus for realizing real time image subtraction in a less complex yet high resolution, good contrast, large dynamic range manner.